The present invention generally relates to processes for producing ruthenium dioxide films, such as for thin-film resistors used in electronic circuits. More particularly, this invention relates to a method for forming a ruthenium dioxide film by laser decomposing a ruthenium-containing precursor.
Resistors formed of ruthenium dioxide (RuO2) are widely recognized in the art for their reliability and stable resistance values. Other notable properties that make ruthenium dioxide of interest for various applications include its high temperature capability, resistance to acids, a hardness similar to fused silica, and electrical conductivity similar to that of ruthenium metal. Ruthenium dioxide also exhibits interesting optical and diffusion barrier properties. As a result, in addition to its use as a resistor material, ruthenium dioxide has been considered as a material for thermistors, electrodes for chlorine production, electrodes for microcapacitors, metallizations for very large scale integration (VLSI), optically transparent electrically conductive coatings, and anti-reflective coatings for photomasks.
Thick-film ruthenium dioxide resistors for hybrid electronic circuits are typically printed on ceramic substrates using inks that also contain an organic vehicle and a glass frit composition. However, a limitation of ruthenium dioxide resistors is that their inks must be fired in oxidizing atmospheres in order to prevent reducing ruthenium dioxide to metallic ruthenium. Various other processes for depositing a ruthenium dioxide film are known, including anodization, chemical vapor deposition (CVD), reactive sputtering, evaporation, thermolysis of precursors, ultrasonic spray pyrolysis, and pulsed laser deposition. However, limitations also exist with each of these processes. For example, anodization requires a precoating of ruthenium, thermolysis reactions require substrate temperatures in excess of 300xc2x0 C., and sputtering techniques require elevated temperatures under vacuum conditions. In some cases, the deposited material also requires a high temperature annealing process. As an example, the thermal decomposition of precursors such as ruthenium chloride (RuCl3.nH2O) and ruthenium (III) nitrosyl salts (e.g., ruthenium (III) nitrosyl nitrate, Ru(NO)(NO3)3) have required annealing at temperatures typically above 300xc2x0 C. to form crystalline ruthenium dioxide. See for example, Newkirk et al., Journal of Catalysis, 11 (1968) 370-377; Jang et al., J. Electrochem. Soc., 134 (1987) 1830-1835; Galizzioli et al., J. Appl. Electrochem., 4 (1974) 57-67; Lodi et al., J. Appl. Electrochem., 8 (1978) 135-143; Ardizzone et al., J. Electrochem. Soc., 136 (1989) 1545-1550; and U.S. Pat. No. 5,358,889 to Emesh et al. However, such thermal treatments exceed the maximum temperature capability of flexible polymeric materials. For example, though flexible substrates made of polyimide have one of the highest temperature ranges for processing, polyimides cannot be subjected to temperatures exceeding about 300xc2x0 C. for extended periods of time.
In view of the above, it can be appreciated that it would be desirable if ruthenium dioxide could be deposited on a substrate without heating the bulk substrate and without annealing the deposited film following deposition. Such a method would enable ruthenium dioxide resistors to be formed on a wide variety of substrates, including non-ceramic materials that otherwise cannot withstand the high processing temperatures conventionally required to fire or anneal ruthenium dioxide films.
The present invention provides a method of forming a ruthenium dioxide film for such purposes as the fabrication of stable thin-film resistors for microcircuits. The method does not require a thermal treatment that heats the bulk substrate on which the ruthenium dioxide film is formed, and is therefore compatible with non-ceramic substrates, e.g., polymeric substrates such as those used as printed circuit boards (PCB) and flexible circuits.
The method generally entails forming an inorganic ruthenium-based film on a substrate, and then thermally decomposing at least a portion of the ruthenium-based film by exposure to a high-intensity beam of radiation to yield a ruthenium dioxide film on the substrate. Particular ruthenium-based precursors suitable for forming the ruthenium-based film include ruthenium (III) chloride (RuCl3.nH2O) and ruthenium (III) nitrosyl salts (e.g., ruthenium (III) nitrosyl nitrate, Ru(NO)(NO3)3). Suitable precursors are generally colored (i.e., light-absorbing), soluble derivatives of ruthenium capable of being thermally decomposed at less than 300xc2x0 C., and therefore also include ruthenocene ((C5H5)2Ru), ruthenium acetate, and triruthenium dodecacarbonyl (Ru3(CO)12). According to an embodiment of the invention, a high-intensity beam of radiation (e.g., visible light and particularly laser light) within an appropriate wavelength range and delivered at an appropriate power level is capable of decomposing certain ruthenium-based compounds to yield ruthenium dioxide without excessively heating the bulk substrate or its surface, yet is capable of completely converting the compounds to ruthenium dioxide that is suitable for use as a thin-film resistor without any subsequent thermal treatments that might damage the substrate. As a result, the present invention enables ruthenium dioxide films to be deposited on flexible polymeric substrates having a limited temperature capability, e.g., less than 300xc2x0 C. With appropriate material selections, the method of this invention is compatible with metallic compositions for conductor traces, so that the ruthenium dioxide resistor can be formed before or after deposition and patterning of terminations for the resistor.
Other objects and advantages of this invention will be better appreciated from the following detailed description.